Techniques of compositing objects, which include transparency, to create a final image are well-known in the art. In a first conventional method, which derives from the "painter's algorithm", each object is normally "rendered" to create a corresponding pixel representation. The pixel representation is normally utilised in building up a final image of the objects through combining it with the representation of other objects. In order to implement transparency, a separate "alpha" channel value is often stored for each pixel of the object, indicating the object's degree of transparency. For a detailed description of combining objects having degrees of transparency, see pages 253 to 259 of an article entitled "Compositing Digital Images", SIGGRAPH 84, written by Porter and Duff, incorporated herein by reference.
An object 1 is shown in FIG. 1 that is to be rendered on top of the object 2 on a computer graphics image processing system. The object 1 is defined to be of a first colour and is partially transparent in that, after rendering, the object 2 is partially visible through the object 1. The border, or outline, of both objects 1 and 2 is defined in terms of splines in accordance with conventional techniques for defining such objects in the art.
The creation of the image of FIG. 1 using the conventional alpha channel method will now be described with reference to FIGS. 2 to 4. In FIG. 2, the currently created image consists of the previously rendered object 2. The object 2 is different from the object 2 of FIG. 1 in that it has been "scan converted", or rendered, to consist of an array of pixels rather than the format of FIG. 1 which consists of a spline outline and a representation of the internal colour of the object 2. In order to composite the object 1 shown in FIG. 3 onto the current state of the image (as defined by FIG. 2) to form the desired image shown in FIG. 4, which includes both of the objects 1 and 2, it is necessary to notionally render the object 1 into a corresponding pixel representation, with each pixel including colour and transparency information, in accordance with the Porter and Duff model, so that the corresponding pixel image of FIG. 3 can be combined with that of FIG. 2 to form the final image as shown in FIG. 4.
In order to render objects 1 or 2, which, as noted previously, are defined by a spline outline, it is normally necessary to convert the spline outlines to an approximation to the spline outline which consists of line segments rather than splines. This process of conversion is commonly known as "vectorisation". The process of vectorisation is well known to those skilled in the art. For example, two methods are described in detail at pages 487 to 488 and 511 to 514 of Computer Graphics: Principles and Practice, written by Foley et al., and published in 1990 by Addison-Wesley Publishing Company, incorporated herein by reference.
Referring now to FIG. 5 there is shown the result of vectorisation of the object 1 into a corresponding polygon having five sides 6 consisting of straight line segments (6A to 6E). The outline of object 1 is such that it "doubles back" on itself and the object 1 overlaps itself so that two portions of the object 1 are present in the overlapping portion 5.
If the object 1 is partially transparent, it is desirable to exhibit a `two and one half` dimensional effect within the portion 5 so that the overlapping becomes more evident to the observer of the final image to be created. However, the foregoing conventional scan line methods are not be able to achieve this effect. They are disadvantageously unable to determine if a current object is overlapping or not.